1. Field of the Invention
The present invention relates to a mobile station and a base station as well as a mobile communication system which uses them, and more particularly to a transmission power control system for controlling the transmission power of a mobile communication system such as a car phone or a portable telephone system.
2. Description of the Related Art
In a cellular system which employs a direct spread code division multiple access (DS-CDMA) system, a plurality of mobile stations use radio waves of the same frequency at the same time to communicate with one or more base stations. In the system of the type mentioned, in order to prevent deterioration of the reception quality by interference, it is essential to equalize the reception levels from different mobile stations at a base station.
Therefore, in mobile communication of the DS-CDMA system, it is required to control the transmission level of each mobile station at a high speed and with a high degree of accuracy over a wide dynamic range in response to a variations of the distance from the mobile station to the base station or to an instantaneous variation by shadowing of the mobile station by a building or a like structure or by multi-paths of the mobile station.
Since the variation of the distance from a mobile station to a base station or shadowing appears also with the reverse-link similarly, it can be overcome to some degree by open loop power control by which the transmission level for the reverse-link of the mobile station is controlled in response to the reception level in the forward-link by the mobile station. However, in order to overcome an instantaneous variation by multi-paths, where different frequency bands are used for the reverse-link and the forward-link, closed loop power control is required by which a control command is sent from the base station to the mobile station in response to the reception level in the reverse-link by the base station and control the transmission level for the reverse-link by the mobile station.
More particularly, according to the closed loop power control, the base station measures the reception level within a time slot of 0.625 ms (milliseconds), and compares the reception level with a reference value to determine whether or not the reception level is equal to or higher than the reference level. Then, when the base station determines that the reception level is equal to or higher than the reference value, it issues an instruction over the forward-link to vary the transmission level of the mobile station by xe2x88x921 dB (decibel). On the contrary, when the base station determines that the reception level is lower than the reference value, it issues an instruction over the forward-link to vary the transmission level of the mobile station by +1 dB. The mobile station thus varies the transmission level in accordance with the instruction for the transmission level variation received from the base station beginning with the time slot after the reception of the instruction.
Such a mobile communication system of the closed loop power control system is disclosed, for example, in Japanese Patent Laid-Open No. 79701/1998, Japanese Patent Laid-Open No. 177488/1999 or Japanese Patent No. 2,974,274.
A disadvantage of the mobile communication system of the closed loop power control system described above is that, when the communication quality by the forward-link is poor, the mobile station is likely to misinterpret the instruction for the transmission level variation received from the base station. In particular, even if the base station issues an instruction (for example, of a transmission series of xe2x80x9c00xe2x80x9d) to vary the transmission level of the mobile station by xe2x88x921 dB (decibel), if the line condition is unfavorable, then the mobile station may recognize that it receives an instruction (for example, a reception series of xe2x80x9c11xe2x80x9d) to vary the transmission level of the mobile station by +1 dB (decibel) and thus vary the transmission level of the mobile station by +1 dB. In this instance, interference in the reverse-link with some mobile stations at the base station may increase and disturb the communication of the base stations with the other mobile station.
On the contrary, if the base station issues an instruction (for example, of a transmission series of xe2x80x9c11xe2x80x9d) to vary the transmission level of the mobile station by +1 dB (decibel), if the line condition is unfavorable, then the mobile station may recognize that it receives an instruction (for example, a reception series of xe2x80x9c00xe2x80x9d) to vary the transmission level of the mobile station by xe2x88x921 dB (decibel) and thus vary the transmission level of the mobile station by xe2x88x921 dB. In this instance, the communication quality in the reverse-link to the base station may be further deteriorated and possibly cause interruption of the communication.
It is an object of the present invention to provide a transmission power control system which can control the transmission level of a mobile station so that the reception condition in the reverse-link may be fixed without the necessity to increase the number of bits of a command for instruction to raise or lower the transmission level or without raising the transmission power of a command.
In order to attain the object described above according to an aspect of the present invention, there is provided a mobile station, comprising means for receiving a transmission power control command transmitted over a forward-link from a base station for making a reception condition of a reverse-link at the base station substantially fixed, a reception quality estimation section for estimating the reception quality of the transmission power control command, and a transmission power control section for controlling the transmission power with a control width varied in response to the reception quality estimated by the reception quality estimation section. With the mobile station, even if the communication condition of the forward-link becomes unfavorable, the transmission power of the mobile station is prevented from being displaced by a great amount in a direction different from the instruction from the base station, and the reception condition from the reverse-link at the base station can be fixed.
According to another aspect of the present invention, there is provided a base station, comprising means for receiving a transmission power control command transmitted over a reverse-link from a mobile station for making a reception condition of a forward-link at the mobile station substantially fixed, a reception quality estimation section for estimating the reception quality of the transmission power control command, and a transmission power control section for controlling the transmission power with a control width varied in response to the reception quality estimated by the reception quality estimation section. With the base station, the transmission level thereof can be controlled so that the reception condition through the forward-link by a mobile station may be fixed.
In both of the mobile station and the base station, the reception quality estimation section may estimate the reception quality of the transmission power control command from a reception level of the transmission power control command. This allows the reception quality of the transmission power control command to be estimated and allows the control width of the transmission power to be determined.
As an alternative, the reception quality estimation section may estimate the reception quality of the transmission power control command from a signal interference rate of the transmission power control command. Also this allows the reception quality of the transmission power control command to be estimated and allows the control width of the transmission power to be determined.
As another alternative, the reception quality estimation section may estimate the reception quality of the transmission power control command from a bit error rate of a slot which includes the transmission power control command. Also this allows the reception quality of the transmission power control command to be estimated and allows the control width of the transmission power to be determined.
As a further alternative, the reception quality estimation section may estimate the reception quality of the transmission power control command from a level of a channel estimation value which is used for demodulation of the transmission power control command. Also this allows the reception quality of the transmission power control command to be estimated and allows the control width of the transmission power to be determined.
According to a further aspect of the present invention, there is provided a mobile communication system, comprising a mobile station, and a base station including a transmission power control command production section for transmitting a transmission power control command for the mobile station in response to the reception quality of a reception signal received over a reverse-link from the mobile station, the mobile station including a reception quality estimation section for estimating the reception quality of the transmission power control command from the base station and a transmission power control section for controlling the transmission power with a control width varied in response to the quality estimated by the reception quality estimation section.
As a still further aspect of the present invention, there is provided a mobile communication system, comprising a base station, and a mobile station including a transmission power control command production section for transmitting a transmission power control command for the base station in response to the reception quality of a reception signal received over a forward-link from the base station, the base station including a reception quality estimation section for estimating the reception quality of the transmission power control command from the mobile station and a transmission power control section for controlling the transmission power with a control width varied in response to the quality estimated by the reception quality estimation section.
With both of the mobile communication system, even if the communication condition of the forward-link or the reverse-link becomes unfavorable, the transmission power of the mobile station or the base station is prevented from being displaced by a great amount in a direction different from the instruction from the base station or the mobile station, and the reception condition from the reverse-link or the forward-link at the base station or the mobile station can be fixed.
In both of the mobile communication system, the reception quality estimation section may estimate the reception quality of the transmission power control command from a reception level of the transmission power control command. This allows the reception quality of the transmission power control command to be estimated and allows the control width of the transmission power to be determined.
As an alternative, the reception quality estimation section may estimate the reception quality of the transmission power control command from a signal interference rate of the transmission power control command. Also this allows the reception quality of the transmission power control command to be estimated and allows the control width of the transmission power to be determined.
As another alternative, the reception quality estimation section may estimate the reception quality of the transmission power control command from a bit error rate of a slot which includes the transmission power control command. Also this allows the reception quality of the transmission power control command to be estimated and allows the control width of the transmission power to be determined.
As a further alternative, the reception quality estimation section may estimate the reception quality of the transmission power control command from a level of a channel estimation value which is used for demodulation of the transmission power control command. Also this allows the reception quality of the transmission power control command to be estimated and allows the control width of the transmission power to be determined.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference symbols.